Cell membranes are not merely passive barriers to diffusion, for they contain enzymes, specific receptors and transport systems. Full understanding of many biologically important processes requires detailed knowledge of these membrane functions. One area in which information is sparse is the specific interaction of transport proteins with cell membranes to effect the entry of a small molecule, e.g. transcobalamin 11-vitamin B12, transferrin-iron, and hemopexin-heme. These systems have essential functions, and rational clinical treatment of states, such as certain nutritional deficiencies and hemolytic episods, requires an understanding of the mechanisms of action of the transport proteins involved. Hemopexin, a serum Beta-glycoprotein, functions in the specific transport of intravascular heme to the liver and thus plays a role in conserving iron as well as in preventing toxic effects of heme. The hemopexin-heme-liver cell system is also an informative model of transport protein-ligand-cell interactions because of the specificity of the interaction of hemopexin with the liver, the information available on the chemistry and biology of hemopexin, and its analogy with other protein-mediated transport processes. Previous work on this project has shown the hemopexin-liver interaction to be rapid, saturable, of high affinity and to require calcium and metabolic energy. Moreover, hemopexin is conserved and recycles from the liver rather than being degraded during its transport function. Hemopexin's function is being studied in several ways. First, the mechanims of interaction of heme-hemopexin with the liver in vivo and with liver cells and liver cell membranes in vitro is being further characterized. Questions of interest include: What are the molecular details of the interaction between heme-hemopexin and its receptor on the heptocyte membrane? How is heme released and how does hemopexin recycle to the circulation? What are the molecular properties of the membrane receptor for hemopexin? Second, we will study the intracellular transport of heme in liver cell, with emphasis on the role of cytoplasmic proteins and subcellular organelles like endocytotic vesicels in the transport of heme to microsomal heme oxygenase.